Contact the Texas Dept of Agriculture in your county. They will help you out.

And to answer your next anticipated question concerning insurance... Yes you do need insurance - contact an agent listed under business insurance, or talk to you agent and he will get you going in the right direction.

The state of Texas does not require a license for fertilizer applications only.
The license requirements enter in effect when an applicator is paid for their services and are using pesticides.
The pesticide inclusion is what necessitates licensing.
Texas has two regulating departments, the Department of Agriculture and the Structural Pest Control Board.
The TDA regs allow for much more freedom that the SPCB regs - the principal difference is that TDA regs allow you, the license holder, to train (with documentation) an employee to apply pesticides and you do not have to be there - just document the application.
SPCB requires every technician to be individually licensed.

Like SWD said, the state of Texas does not require a license to apply only fertilizer, just make sure what you are applying does not contain herbicide or insecticide mixed in. This would require a state license to apply.